Among conventional power supply devices incorporated into an electrical apparatus, electronic apparatus, or the like and configured to supply relatively large power to a load are those which use a two-converter circuit in order to reduce standby power. For example, a two-converter circuit includes a main power supply circuit that supplies power to a load which consumes much power and a sub-power supply circuit that supplies power to a less power-consuming load, such as a control circuit, and operates the main and sub-power supply circuits in normal times and operates only the sub-power supply circuit during standby to reduce standby power. However, a two-converter circuit must include two power supply circuits and thus disadvantageously increases the size of the power supply apparatus and increases the cost.
For this reason, there is disclosed a switching power supply device that includes a single main power supply circuit as a power supply circuit and controls a transistor of a switching unit in the power supply circuit so that the transistor operates continuously when the load current is large; and the transistor operates intermittently when the load current is small (see Patent Literature 1).
Examples of a switching power supply device include those of flyback type and those of current resonant type. A current resonant power supply device is used, for example, when it is desired to increase efficiency when a heavy load is used, when it is desired to handle a large output current, or when it is desired to improve noise resistance.